This invention relates generally to semiconductor fabrication techniques and, more particularly, to processes for fabricating low contact resistance ohmic contacts in semiconductors utilizing a compound semiconductor substrate, such as gallium arsenide (GaAs).
Field effect transistors commonly referred to as FETs, are very often fabricated on a substrate of a compound semi-conductor, material, such as gallium arsenide. For example, metal semiconductor field effect transistors (MESFETs) can be fabricated on a gallium arsenide substrate. In one manufacturing process, the substrate is implanted with silicon ions to form an n-type layer along one surface of the substrate. When a conducting metal, such as gold, is deposited on the n-type layer, a rectifying junction is formed, i.e., one which has a desired non-linear relationship between voltage and current. For the formation of a basic field effect transistor, one rectifying contact is required, usually referred to as the gate terminal, and two nonrectifying or ohmic contacts must be made with other regions of the n-type layer. In the past, the ohmic contacts have been made by means of an alloy of gold and germanium, which is deposited on the ohmic contact areas and then heated to a prescribed temperature for a prescribed time. The alloy penetrates the n-type layer and makes relatively good ohmic contact. However, these steps of forming the ohmic contact are inconsistent with the other processing steps in the typical MESFET manufacturing process, which may be employed to manufacture large numbers of MESFET transistors on a single substrate. Moreover, alloy contacts have a tendency to fail after a period of years, especially under high-temperature conditions.
It will be appreciated, therefore, that there has been a significant need for a process for fabricating ohmic contacts on such devices, wherein the steps of the process are not inconsistent with the fabrication steps for the device as a whole. The present invention fulfills this need.